docs: def _load_at(self, time, channels=None):
          """Load a waveform at a given time."""
          if channels is None:
              channels = slice(None, None, None)
          time = int(time)
          time_o = time
          ns = self.n_samples_trace
          if not (0 <= time_o < ns):
              raise ValueError("Invalid time {0:d}/{1:d}.".format(time_o, ns))
          slice_extract = _slice(time_o,
                                 self.n_samples_before_after,
                                 self._filter_margin)
          extract = self._traces[slice_extract][:, channels].astype(np.float32)
  
          # Pad the extracted chunk if needed.
          if slice_extract.start <= 0:
              extract = _pad(extract, self._n_samples_extract, 'left')
          elif slice_extract.stop >= ns - 1:
              extract = _pad(extract, self._n_samples_extract, 'right')
  
          assert extract.shape[0] == self._n_samples_extract
          return extract